The present invention relates to electrodes for use in batteries. More particularly, the invention relates to nickel-oxide electrodes capable of being electrically recharged which are particularly useful in batteries having a nickel-cadmium; nickel-zinc, or nickel-hydrogen couple.
Nickel-oxide electrodes have been used in various types of batteries, and particularly rechargeable energy-storage batteries including nickel-cadmium, nickel-zinc, and nickel-hydrogen couples. During the energy producing phase of the cycle, i.e., discharge of the battery, the nickel electrode acts as the positive or oxidizing electrode. The battery can be recharged by applying energy. From such use, it is recognized that the nickel-oxide electrode is advantageous in view of its (1) high oxidizing potential; (2) high degree of reversibility, i.e., the electrode can be efficiently discharged and charged at high rates; and (3) low solubility in strong alkaline electrolytes which provides the basis for a long cycle life.
Accordingly, considerable interest has been developed in the nickel-oxide electrode in recent years for use in mobile energy-storage applications when combined with the zinc or hydrogen negative couples. However, for mobile energy-storage applications, such as electrical vehicle use, cost per kilowatt, cost per kilowatt hour, volume and weight per kilowatt, and volume and weight per kilowatt hour become important factors. The presently available nickel-oxide electrodes have been found to be inadequate to permit widescale use in mobile energy-storage applications from both cost and performance parameters.
More specifically, the nickel-oxide electrodes most widely used heretofore comprise a porous, usually sintered, nickel plaque as the current collecting substrate and support for the active nickel-oxide phase. The sintered nickel plaques are manufactured utilizing powder metallurgy processes which include either pressing dry nickel powder onto both sides of an expanded nickel sheet, or slurry coating an expanded nickel sheet with a suspension of nickel powder. The green compacts are then sintered in hydrogen atmospheres to produce the nickel plaques. The plaques are subsequently loaded with nickel hydroxide by vacuum impregnation or by electrochemical deposition or the like processes. When using an electrochemical deposition process, the porous plaque is polarized cathodically in an acidic, concentrated (2 molar) solution of nickel nitrate. As hydrogen ions are discharged on the nickel surface the pH increases inside the plaque and nickel hydroxide is precipitated on the nickel substrate. When the pores have been substantially filled with nickel hydroxide, the material is oxidized to the higher valency state by reversing polarity in alkaline solution and the plaque is then in an activated oxidized state ready for discharge.
There are recognized restraints and/or limitations in the processes as above noted for making nickel-oxide electrodes and in the performance characteristics and cost parameters of the resultant electrodes. One such restraint and limitation arises from a tradeoff between plaque porosity and mechanical strength. In order to obtain the highest possible loading of active nickel oxide, porosity should be as high as possible and the average pore size at least about 10 microns. However, because of the limitations on the nickel powders available, porous plaques having a porosity of greater than about 75 to 80 percent, at least to the extent of economic practicality, have inadequate mechanical strength for use as electrode structures. Additionally, with available nickel powders the pore size obtainable in the electrodes produced is relatively small, again from the standpoint of economic practicality. A further limitation is the inability to make a plaque thicker than about 0.040 inch based on the presently available materials and technology. This is because the thickness of the plaque is dictated primarily by two factors, (1) mechanical strength, it being recognized that as thickness of the sintered plaque increases the more fragile the plaque becomes; and (2) uniform loading of the sinter, it being recognized that as the sintered plaques increase in thickness it becomes progressively more difficult to uniformly load the interior of the plaque with the active nickel hydroxide. Moreover, when the electrode plaque is made by slurry coating of an expanded nickel sheet, the viscosity of the nickel powder slurry again limits the thickness of the plaque to no more than about 0.040 inch and, additionally, a porosity above about 75 to 80 percent is virtually impossible or impractical to obtain.
Since electrode thickness and high porosity are important cost parameters in making a commercially feasible electrode in that thick highly porous electrodes have larger capacities per unit area and, therefore, fewer electrodes are required to construct a battery of given ampere-hour capacity, attempts have been made to provide thick nickel-oxide electrode structures by combining conducting carbon particles with nickel-oxide powder and binding the materials together around a nickel expanded metal conductor with organic and/or inorganic binders stable in an electrolyte medium. The most commonly used binder has been polytetrafluoroethylene (PTFE). Although such electrodes have been successfully demonstrated, they have proven inadequate in terms of mechanical life and material cost to be completely practical.